First thing I'm sorry if this question sounds too much beginner-ish. I don't major in electrical engineering so I may lack the basic knowledge.
I have been noticing that, while tuning my AM radio, nearly all stations have a "secondary frequency" that transmits the same audio as the advertised frequency. However, the quality of the audio is a little lower than the advertised frequency. For instance, the channel's advertised frequency is 675 kHz and I found the secondary frequency at 630 kHz. Another example is that main frequency at 549 kHz and secondary frequency at 729 kHz.
The pairs of main-secondary frequency do not seem to follow any patterns or rules. So my question is:
- Is this on purpose (as a backup frequency or something else)?
- If it's not on purpose, why does this happen?
More information: My radio is a basic consumer radio (Sony) with FM and AM (AM radio tuner is ranged from 531 kHz to 1602 kHz, with 9 kHz step between frequencies).